The present invention relates specifically to an artificial satellite, which is usable in the field of communications, such as satellite communications and mobile communications, a satellite orbit control method and a communication system using the satellite.
There is a requirement to transfer medical information including image data, for an emergency case carried on an ambulance from the ambulance to a paramedic center, and to direct medical treatment suitable for the emergency case to the ambulance from the doctors on duty at the paramedic center.
However, in the case of trying to transfer large-sized data, like image files, satisfactory results can not obtained by the conventional ground-base communication infrastructure. In addition, in case of communicating via geostationary satellites currently in service and/or mobile communication satellites to be deployed in the future, it is difficult to establish stable and continuous transfer lines extended from moving bodies because of shielding objects such as building structures and trees.
Though it is certainly possible to transfer large-sized data from movable objects, like an automobile, by using satellites moving in a specific orbit, no definite method for defining such an orbit has been established to date. Therefore, the orbit-related elements of such a specific orbit have not been definitely identified as yet.
Conventional technologies and their problems are described below in detail.
(A) Technologies and Problems in Existing Communication Infrastructures
(A-1) Technologies and Problems in Ground-base Communication Infrastructures
In a case where large-sized data, like image files, are transferred from the movable bodies like automobile to a ground-base fixed station, communication methods via ground-base communication infrastructures or communication satellites can be considered. However, existing communication methods may not satisfy all the requirements for the system specification and performance.
Now, let's take as an example an ambulance. For carrying an emergency case by ambulance, the average carrying time period is about 27 minutes. For serious cases, there occurs many instances in which the emergency case may die if adequate medical treatment is not applied in time, which is a strong motivation for the medical specialist to apply medical treatment to the emergency case in the ambulance or to suggest an adequate method for medical treatment to the emergency case to the emergency medical technician in the ambulance. However, about 15,000 or more medical doctors would be required for paramedic services in order to dispatch medical doctors with shift work to the about 5,000 ambulances in Japan. However, this is not realistic, and so it is considered to be more effective to communicate adequate methods for medical treatment from the paramedic center to the ambulance. However, in the conventional ground-base communication systems, communication lines with phone-level quality with which an instantaneous break may occur frequently are only available, and therefore, adequate methods for the medical treatment can not be directed satisfactorily from the paramedic center. If image information captured by endoscope, electrocardiogram, echo and camera could be directly transferred to the paramedic center, it is supposed that satisfactory diagnosis and directions for medical treatment of the emergency case could be given. However, ground-base communication infrastructures have such problems as limitation of transmission band, limitation of communication coverage areas, cross talk and interference due to reflection by man-made building structures, and so can not be applied to practical use for such a purpose.
Similarly, though many requests exist for large-scale data transmission from movable bodies, for example, live telecast of a marathon, the conventional ground-base communication infrastructure can not be used for this application.
(A-2) Technologies and Problems in a Geostationary Communication Satellite System
In the field of satellite communications using artificial satellites, communication systems using geostationary satellites and low-to-middle altitude orbits are well known. There are the following problems in conventional communication satellites.
As a geostationary satellite has about a 24-hour orbit cycle almost equal to the earth's rotation cycle, the geostationary satellite can be viewed from the ground to be stationary at a point above the Equator. However, the elevation angle of such a geostationary satellite is low, for example, the elevation angle at Tokyo is at most 45 degrees even in case of good conditions. As the movable bodies in metropolitan areas move on the roads surrounded by artificial building structures and roadside trees, the lower range of the elevation angle is blocked by those obstacles, and satellite communications with geostationary satellites may be blocked. As the stationary satellites can be seen in an east-south to west-south direction, though communication lines can be established in a case where the movable body moves in a north-to-south direction and a broader visual field to the satellite can be obtained, communication lines may be blocked by building structures and roadside trees at almost any time in a day in a case where the movable body moves in an east-to-west direction, especially, in a west direction. Therefore, satellite communications using qeostationary satellites do not produce satisfactory results for service in not-plain areas, like a metropolitan area and a mountain area.
(B) Technologies and Problems of Satellite Communication Systems Currently Under R&D
In the case of satellite communication systems using low-to-middle altitude orbits, such as Iridium and Odyssey currently under development for the purpose of cellular phone services using mobile communication satellites, the duration of time while the satellite in service stays within a high elevation angle range and comes in sight from the ground is generally short due to the limitation on the number of orbital planes for the satellite and the number of satellites in service. Especially, since a satellite flying on the low altitude orbit has about 90 to 100 minutes in its orbit cycle, the duration of time while the satellite stays within a high elevation angle range as viewed from the ground is as short as a few minutes. Therefore, when trying to use or apply this kind of satellite communication systems for the purpose of stable and definite communication for large-scale data, as used in the above example of an ambulance and a paramedic service system, without any influence by building structures, plants and natural topographic features over a certain extended time period, for example, more than 27 minutes, it is required to configure such a system using plural satellites which alternately may come in sight at a higher elevation angle. In this case, some thousand or more satellites are required, which causes difficulties in procuring a number of satellites, the operation thereof and launching cost reduction, and so this plan is not practical also from an economical point of view.
In the case where a higher elevation angle is required, as in the above example, conventional geostationary satellites for practical use and low-to-middle altitude satellites currently under development are not fully applicable.
(C) Technologies and Problems in a Satellite Communication System Currently Under Study
For example, as found in research reports, such as "Feasibility of Mobile Communication Mission Using NonGeostationary Satellite Orbits", Technical Research Report, Japanese Electronics, Information and Communication Society, Vol. 89, No.57, satellite communication systems currently understudy are discussed. Especially, an oblong orbit having a larger eccentricity squared is proposed in some research reports including the above report.
According to Kepler's Law, an object passing around the apogee point of the orbit slows down. By defining an orbit having its apogee point located on the upper air of the target service area, the duration time during which the satellite on this orbit stays at a high elevation angle can be taken to be long enough. Therefore, it is necessary to use an oblong orbit in order to establish communication lines for a extended period of time without a communication break due to building structures, roadside trees and natural geographical conditions.
As an example of oblong orbits, the Molnia orbit having about a 12-hour orbit cycle, a perigee altitude of some hundred km and an orbital inclination angle of about 63.4 degrees has been practically used as an orbit for communication satellites and military satellites in Russian territory since the 1960's. Though this orbit is a stable orbit with its argument of perigee being fixed, and is certainly practical for service at the higher latitude locations over the Russian territory extended in a north and south direction, this orbit is not so practical for service at the lower latitude locations extended in a north and south direction, such as over Japan. Some orbits having about an 8-hour orbit cycle, about a 12-hour orbit cycle and about a 24-hour orbit cycle are proposed for the services provided in the Japanese territory. However, those proposed orbits are designed with localized optimization, and, as the orbits suitable for the north-to-south and east-to-west extension of the Japanese territory, there has not been any proposal for optimized orbits, methods for defining those orbits and definite operation technologies. This is because the design methodology for definition of a satellite orbit has been empirical in order to determine six orbit-related elements.
There are various methodologies for identifying and defining orbits, but the following six orbit-related elements are mainly used. Those are defined for an individual reference time.
Semi-Major Axis a: semi-major axis of the ellipse (noted by symbol 54 in FIG. 5), PA0 Eccentricity Squared e: flatness of the ellipse orbital PA0 Inclination Angle I: angle defined between the orbital plane and the equational plate PA0 Right Ascension of North-Bound Node .OMEGA.: angle (shown by symbol 63 in FIG. 6) measured in the east direction from the vernal equinoctial point to the crossing point of the orbit from the northern hemisphere to the southern hemisphere with the equational plate (this crossing point shown by symbol 62 shown in FIG. 6) PA0 Argument of Perigee .omega.: angle measured between the perigee and the right ascension of north-bound node 62 on the orbital plane (shown by symbol 63 in FIG. 6) (0 degree.ltoreq..omega..ltoreq.360 degrees) PA0 True Anomaly .theta.: angle defined by the line connected between the perigee and the focal point of the ellipse and the line connected between the satellite and the focal point of the ellipse (shown by symbol 58 in FIG. 5)
(0 degree.ltoreq..OMEGA..ltoreq.360 degrees) PA1 (0 degrees.ltoreq..theta..ltoreq.360 degrees).
The geometrical relationship for those elements will be described with reference to FIGS. 5 and 6. The satellite 51 moves on the elliptical orbit having a focal point 50. The distance between the perigee 53 of the ellipse and the focal point 50 of the ellipse is represented by perigee radius Rp and with symbol 57 in FIG. 5. The distance between the apogee 52 of the ellipse and the focal point 50 of the ellipse is represented by apogee radius Ra and with symbol 56 in FIG. 5. Perigee radius, apogee radius, semi-major axis a represented by symbol 54 in FIG. 5, semi-minor axis b represented by symbol 55 in FIG. 5 and the eccentricity squared e have the following relations. EQU Rp=a(1-e)
Ra=a(1+e) EQU B=a(1-e.sup.2)1/2 EQU e=(Ra-Rp)/(Ra+Rp)
In FIG. 6, what is shown is an example in which the earth 60 is positioned at the focal point of the elliptical orbit. The elliptical orbit crosses at the north-bound node 62 on the equational plate from the southern hemisphere to the northern hemisphere, while the perigee is positioned at the point 65 and the apogee is positioned at the point 66. The angle 64 between the equational plate 61 and the orbital plane defines the orbital inclination angle i. The right ascension of the north-bound node is defined by the angle 68 measured in the eastern direction from the vernal equinoctial point, and the argument of the perigee is defined by the angle 63 between the north-bound node 62 and the perigee 65.
Even if the semi-major axis can be specified definitely by the orbit cycle, other major parameters may be determined to be arbitrary values, such as the eccentricity squared is an arbitrary real number 0.0 or over and less than 1.0, the orbital inclination angle is an arbitrary real number 0.0 degree or over and 180 degrees or smaller, and the argument of perigee is an arbitrary real number 0.0 degree or over and 360 degrees or smaller. Thus, there may occur a situation in which a designer is forced to determine values for those parameters intuitively and/or empirically from his or her experiences.
If a satellite which can come in sight in the zenith direction for an extended period of time on the upper air of the target service area can be realized, "large-scale data transfer from mobile bodies for an extended period of time" can be established by satellite communications. Thus, what has been sought are feasible methodologies for defining orbit-related elements and their definite values which can be adaptive to Japanese territory characteristics and are cost-effective, that is, configured with less number of satellites forming the overall system.
As described above, in order to transfer large-scale data including image files from movable bodies, like an automobile, for an extended period of time, it is required to make the satellite remain on the orbit in the zenith direction as long as possible and to communicate with the satellite.
It has been generally recognized that it is preferable to establish an orbit shaped in an oblong ellipse having its apogee on the upper air of the target service area, in order to satisfy the above described requirement. However, adequate methodologies and algorithms for defining orbit-related elements have not been proposed. In addition, there is no definite proposal for specified values for those parameters to be optimized for the services over the whole Japanese land.